Double Refraction

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Double Refraction Definition

The double refraction of light is the phenomenon of birefringence. It is an optical property in which a single ray of unpolarized light enters an anisotropic medium and splits into two rays, each travelling in a different direction.

We can think of double refraction as the end which divides into two roads. Here, the end is the anisotropic medium, the person travelling is the unpolarized light, while the two roads are the two rays, each travelling their paths. 

In this article, we will understand the definition of double refraction, explain the phenomenon of double refraction in depth.


Explain Double Refraction

We observe that in double refraction light is unpolarized and it divides into two rays when passed through the doubly refracting crystal that deviates the rays into different directions.

We can also observe the Double refraction of light by comparing two materials, viz: glass and calcite. 

If a mark is drawn upon a sheet of paper with a pencil and then covered with a piece of glass, only one image is visible; but if the same paper is covered with a piece of calcite, and the crystal is adjusted in a specific direction, then two marks are visible.


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The figure above shows the phenomenon of double refraction via calcite crystal. An incident ray is seen to split into two rays, the first is the ordinary ray CO and another is the extraordinary ray, i.e., CE on entering the crystal face at point C. 


Do you Know?

If the incident ray enters along the direction of its optic axis of the calcite crystal, the unpolarized light doesn’t split into rays; however, it is necessary for light to enter the crystal at a certain angle.

Now, we will understand the phenomenon of double refraction:


Explain the Phenomenon of Double Refraction

The optic axis of a Calcite crystal (doubly refracting crystal) is defined by the symmetry of the crystal lattice. In calcite compounds or CaCO3, the CO3 (Carbon trioxide) forms a triangular cluster and the optic axis lies perpendicular to this. 

When light enters along with the optic axis of the crystal, nothing happens and the light comes out unpolarized. However, when the light enters at a certain angle to the optic axis, the asymmetry of the lattice splits the ray into two with mutually orthogonal polarizations, as shown in the below diagram of Birefringence in a calcite crystal. 

From the below figure, we see that one ray is the Ordinary ray, for which Snell's law holds, while the other is the Extraordinary ray that does not obey Snell's law.


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Diagram: Birefringence in a calcite crystal

Observation

On observing an object through the above crystal, we see a double image of the object. While analysis through the Polaroid sheet shows that these images have axes of polarization perpendicular to each other; therefore, rotating the Polaroid makes the images alternately disappear. 

Things get interesting when you place a second crystal just on the top of the first. Now, you have four images instead of two, but when you rotate it, the second crystal functions as an analyzer for the first one, and you get two images again.


Double Refraction of Light

In double refraction of light, the ordinary ray and the extraordinary ray are polarized in planes oscillating at right angles to each other. Furthermore, the refractive index, i.e a number that determines the angle of bending specific for each material medium of the ordinary ray that is observed to be constant in all directions.

The refractive index of the extraordinary ray changes according to the direction taken because it has both parallel and perpendicular components to the crystal’s optic axis. 

It’s because the speed of light waves in a medium that is equal to their speed in a vacuum is divided by the index of refraction for that wavelength, an extraordinary ray can move both faster and slow than an ordinary ray.


Do You Know?

All transparent crystals like calcite crystals except those of the cubic system that is normally optically isotropic possess the phenomenon of double refraction: in addition to calcite, some well-known examples are sugar crystals, ice, mica, quartz, and tourmaline. 

Other materials may become birefringent under special circumstances. Now, let’s consider some examples for the same:

  1. Solutions of long-chain molecules exhibit double refraction when they flow, and this principle is called streaming birefringence.

  2. Plastic materials formed of long-chain polymer molecules can also become doubly refractive when compressed or stretched. This phenomenon is called photo elasticity. 

  3. There are some isotropic materials like glass that also exhibit birefringence when placed in a magnetic field or electric field or when exposed to external stress.

Enhancing Knowledge!

The effect of birefringence was first described by the Danish scientist named Rasmus Bartholin in 1669.

FAQs (Frequently Asked Questions)

1.  Explain Doubly Refracting Crystals.

Ans: The property possessed by certain crystals that are capable of forming two refracted rays from a single incident ray is a doubly refracting crystal. 

Here, the ordinary ray follows the laws of refraction. On the other hand, the refracted ray or the extraordinary ray follows different laws. The light in the ordinary ray gets polarized perpendicularly to the light in the extraordinary ray. 

Along an optic axis of the crystal (notable calcite), the ordinary and extraordinary rays travel with the same speed. 

Some crystals, like calcite, quartz, and tourmaline, have only one optic axis; that’s why they are uniaxial crystals. While materials like mica and selenite, have two optic axes; and they are called biaxial crystals; this phenomenon is also known as birefringence and the doubly refracting crystal is called a birefringent crystal. 

2. What is Birefringence?

Ans: Birefringence is the optical property of a material having some refractive index value that relies on the polarization and the direction of propagation of light; these optically anisotropic materials are said to be birefringent (or a bi refractive). 

The birefringence is numerically the maximum difference between refractive indices possessed by the material. Crystals with non-cubic crystal structures are birefringent, akin to plastics under mechanical stress.

Birefringence is responsible for the phenomenon of double refraction of light.

It is a phenomenon in which a ray of light, when incident upon a birefringent material, gets split by polarization into two rays going in slightly different paths.

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